1. Introduction
This invention relates to an etchant comprising sulfuric acid activated with hydrogen peroxide and characterized by an enhanced etch rate. The etchant is particularly useful in the manufacture of printing circuit boards.
2. Description of the Prior Art
The activation of relatively dilute mineral acids with an oxygen release compound is known in the art and described, for example, in Plating "Surface Treatment of Metals with Peroxygen compounds", Vol. 42, p. 561 (1955). In the Plating publication, it is taught that mineral acids such as sulfuric acid, nitric acid, acetic acid and phosphoric acid may be activated with oxygen release compounds such as hydrogen peroxides or other peroxy, persulfate or perborate compounds. The oxygen release compounds enhance the etch rate thereby permitting a decrease in the concentration of the acid.
Activation of dilute sulfuric acid with hydrogen peroxide is most attractive due to low cost and ease of electrolytic copper recovery from the spent etchant prior to disposal. However, the utility of hydrogen peroxide etching solutions is reduced somewhat due to catalyzed decomposition of hydrogen peroxide caused by etched metal ions or other transition metal ions in solution and a slow etch rate, typically 1.4 mils of copper from 1 ounce copper clad laminate in 10-25 minutes at 120.degree. F. To enhance or exalt etch rate, more concentrated solutions of hydrogen peroxide have been used, but concentrated peroxide solutions are hazardous to health and safety. In addition, such etchants have not been successfully used with tin dissimilar metal etch resists because they attack tin, especially immersion tin.
In U.S. Pat. No. 4,130,454, incorporated herein by reference, a modified peroxide etch is disclosed and claimed. The etchant comprises hydrogen peroxide and a molybdenum compound in an acidic solution. The hydrogen peroxide provides a sustained etch rate and the molybdenum etches at a substantially greater rate than the peroxide thereby exalting the rate to a desired level for commercial use. Molybdenum cannot be used as a sole oxidant because it is rapidly depleted due to reduction to a lower valence form as etching proceeds. It is therefore incapable of providing sustained etching. Patentee proposes a theory that the combination of peroxide and molybdenum is a synergistic combination because both are believed to etch the peroxide in the acid environment while providing the secondary function of oxidizing molybdenum to a higher valence capable of etching metals in the acidified environment.
The peroxide etchants described above are used to etch copper both by immersion and by spraying. Immersion etching comprises immersing a part within the etchant until the desired amount of metal is dissolved. Spray etching comprises spraying a finely divided stream of etchant against the surface of the part until the desired amount of metal is dissolved. Spray etching generally provides the advantage of a more rapid etch rate because the finely divided droplets of etchant are aerated resulting in additional oxygen at the surface of the part being etched and a concommitant increase in etch rate.
The etchants of the aforesaid U.S. Pat. No. 4,130,454 do provide the exalted etch rate as reported in the patent for immersion etching. However, for spray etching, the etch rate appears to be retarded rather than exalted which is contrary to that which would be expected. Consequently, the etchant of said patent is usable for spray etching, but spray etching of said etchant may not be commercially practical where high through put of parts is required.